This invention generally relates to ion beam sources, and, in particular, to generating highly collimated ion beamlets by utilizing a set of grid optics in an ion beam source.
Ion beams come in many varieties and have many industrial applications. For example, a collimated low-power ion beam may be employed to align inorganic materials in a liquid crystal display. Alternatively, a collimated high-power ion beam may be employed to ion etch a surface or propel a vehicle in space. A highly collimated ion beam, which has minimum off-axis velocity components, is generally desirable in many broad beam ion source applications. Other applications of collimated ion beams include without limitation ion milling and aligning superconducting YBa2Cu3O7 crystals as they grow using Ion Beam Assisted Deposition techniques to increase the current carrying capability of High Temperature Superconducting tapes.
An xe2x80x9cionxe2x80x9d is a charged particle, referring to any atom or molecule having an unbalanced electrical charge, which is a result of having lost or gained one or more electrons. A gas phase collection of ionized atoms, molecules, and/or electrons is referred to as xe2x80x9cplasmaxe2x80x9d. Generally, plasma states can be induced in gases, such as neon, argon, etc., by applying high-energy radio frequency (RF) fields or a direct current (DC) voltage to the gaseous matter. The radio frequency fields remove electrons from the particles, giving the particles a positive charge. Using electric fields, the ions can then be extracted from the plasma and propelled to a target in an ion beam.
One technique for extracting ions from the plasma involves placing an electrical field near the plasma. A simple example of an ion beam provides plasma in a plasma chamber with two electrified grids positioned at an opening of the plasma chamber. Each grid has an array of apertures to allow ions to travel through the grid during operation. Typically, the apertures of one grid are closely aligned with apertures of the other grid. The first electrified grid (i.e., the grid closest to the plasma) is called an xe2x80x9cextraction gridxe2x80x9d or a xe2x80x9cscreen gridxe2x80x9d, and has a high positive electrical potential (e.g., 1000 Volts). The second grid, called an xe2x80x9cacceleration gridxe2x80x9d, is spaced closely to the first grid and has a negative potential (e.g., xe2x88x92400 Volts).
For 2-grid systems, ion beam divergence is strongly dependent upon normalized perveance per aperture, the extraction-grid-to-acceleration-grid spacing, the aperture size, and the net-to-total-accelerating-potential ratio. Perveance is a normalized measure of the current of ions extracted through each aperture. Adjustments to the spacing and aperture hole sizes can reduce the net ions impinging upon the grids and decrease the angular divergence of the ion beam. It has been shown that the best-case divergence angle for a two-grid ion source is 10 degrees and can be as large as 30 degrees. See G. Aston et al, xe2x80x9cIon Beam Divergence Characteristics of Two-Grid Accelerator Systemsxe2x80x9d, AIAA Journal, Vol. 16, No. 5, May 1978, pp. 516-524; G. Aston et al., xe2x80x9cThe Ion-Optics of a Two-Grid Electron Bombardment Thruster,xe2x80x9d AIAA Paper 76-1029, Key Biscayne, Fla., 1976; Y. Hayakawa et al., Ion Beamlet Divergence Characteristics of Three-Grid Accelerator Systems,xe2x80x9d AIAA Paper 97-3195, Seattle, Wash., 1997.
Another technique involves a third grid, called a xe2x80x9cshield gridxe2x80x9d, which is placed the furthest away from the plasma chamber and is typically spaced closely to the acceleration grid. In many applications, an RF excited plasma bridge neutralizer is positioned in the vicinity of the ion beam output and is used to provide electrons for current and space charge neutralization of the ion beam, for example, to reduce inter-ion repulsion within the ion beam. The shield grid is typically charged to a low electrical potential (e.g., 0 Volts). By positioning the shield grid downstream (i.e., away from the plasma source) of the acceleration grid and operating it at a near ground potential, the neutralization plane becomes fixed in close proximity to the shield grid potential. This characteristic allows for flatter equipotential surfaces between the neutralization plane and the acceleration grid apertures as compared to the 2-grid system, resulting in less off axis divergence of the ion beam (e.g., the best case is about 8.2 degrees).
However, some applications require less off-axis divergence. In an attempt to decrease off-axis divergence, one existing technique has introduced a second positive electrical potential acceleration grid between the extraction grid and the negative-potential acceleration grid described previously. See PCT Application PCT/GB97/02923, ION GUN, by Nordiko Ltd., published Apr. 30 1998. The new acceleration grid is intended to provide xe2x80x9cgentlerxe2x80x9d acceleration so as to allow formation of a lower current, more stably collimated beam, which is less susceptible to space charge forces. However, the existing 4-grid optics system requires the new acceleration grid to be contoured (instead of flat), resulting in non-uniform spacing from center-to-edge between the grids in the ion beam source. Moreover, the main result of xe2x80x9cgentlexe2x80x9d extraction of the ions by the existing 4-grid optics system is low perveance and, therefore, a reduced ion beam current caused by the reduced electrical potential between the extraction grid and the new acceleration grid, which is predicted by Child""s Law. See Child, C. D., Physical Review, Vol. 32, 1911, pp. 492-511. No experimental data is available to support the claim of improved collimation in any existing 4-grid optics system and, furthermore, the reduced ion beam current is inadequate for many applications. Accordingly, existing ion beam systems fail to provide adequate collimation and ion beam current for many applications.
Against this backdrop, the present invention has been developed. The present invention relates to a multi-grid ion beam source having a focus grid positioned between the acceleration grid and the shield grid or between two shield grids. In one embodiment, the focus grid has a large positive potential, resulting in an off-axis divergence of about 1.5 degrees. In another embodiment, the focus grid has a large negative potential, resulting in an off-axis divergence of about 1.4 degrees. The focus grid acts to change momentum of the ions, focusing them into a more collimated ion beam when propelled through a shield grid than previous approaches.
In one embodiment of the present invention, a method of tuning a focus grid potential in an ion beam source is provided. The ion beam source is capable of generating a substantially collimated ion beam that sends ions toward a target along an axis extending between a plasma source and the target. An extraction grid is spaced downstream from the plasma source along the axis and substantially normal to the axis. The extraction grid has a positive electrical extraction potential. An acceleration grid is spaced downstream from the extraction grid along the axis between the extraction grid and the target and substantially normal to the axis. The acceleration grid has a non-positive electrical acceleration potential. A focus grid is spaced downstream from the acceleration grid along and substantially normal to the axis between the acceleration grid and the target to focus the ions exiting the acceleration grid into a substantially collimated ion beam along the axis. The focus grid has an electrical focus potential. A shield grid is spaced downstream from the focus grid along and substantially normal to the axis between the focus grid and the target. The shield grid has an electrical shield potential. The angle of divergence of the ion beam is detected. The electrical focus potential is altered until the detected angle of divergence of the ion beam is minimized.
In another implementation of the present invention, a method of generating a substantially collimated ion beam sending ions toward a target along an axis extending between a plasma source and the target is provided. A positive electrical extraction potential is applied to an extraction grid spaced downstream from the plasma source along the axis and substantially normal to the axis. A non-positive electrical extraction potential is applied to an acceleration grid spaced downstream from the extraction grid along the axis between the extraction grid and the target and substantially normal to the axis. The electrical extraction potential and the electrical acceleration potential operate in combination to extract the ions from the plasma source. An electrical focus potential is applied to a focus grid spaced downstream from the acceleration grid along and substantially normal to the axis between the acceleration grid and the target to focus the ions exiting the acceleration grid by changing momentum of the ions along the axis. An electrical shield potential is applied to a shield grid spaced downstream from the focus grid along and substantially normal to the axis between the focus grid and the target to locate a neutralization plane near the shield grid.
In yet another embodiment of the present invention, an ion beam source for generating a substantially collimated ion beam sending ions toward a target along an axis extending between a plasma source and the target is provided. An extraction grid is spaced downstream from the plasma source along the axis and substantially normal to the axis. The extraction grid has a positive electrical extraction potential. An acceleration grid is spaced downstream from the extraction grid along the axis between the extraction grid and the target and substantially normal to the axis. The acceleration grid has a non-positive electrical acceleration potential, the electrical extraction potential and the electrical acceleration potential operating in combination to extract the ions from the plasma source. A focus grid is spaced downstream from the acceleration grid along and substantially normal to the axis between the acceleration grid and the target to focus the ions exiting the acceleration grid by changing momentum of the ions along the axis. A shield grid is spaced downstream from the focus grid along and substantially normal to the axis between the focus grid and the target to locate a neutralization plane near the shield grid.
These and various other features as well as other advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.